A polycrystalline silicon thin-film transistor is known which uses a polycrystalline silicon film formed on the surface of an insulation substrate as a semiconductor layer, instead of a conventional semiconductor substrate. In such a polycrystalline silicon thin-film transistor, it is desirable to uniformly form polycrystalline silicon of a grain size as large as possible, since the grain boundary of silicon limits mobility of carriers. In a so called laser recrystallization method, in which amorphous silicon as a starting material is heated and melted by laser irradiation and then cooled down for recrystallization, however, it was difficult to control the temperature of melted silicon which promotes crystal growth. Thus, it was difficult to form crystal of a large grain size uniformly and stably.
Meanwhile, there is a technique disclosed in Japanese Patent Laying-Open No. 2000-286195 (Patent Document 1) In Patent Document 1, a visible light laser, Nd:YAG2ω laser beam is focused into a thin line shaped laser beam to have light intensity distribution of an approximately Gaussian shape in the width direction, and is applied as a laser beam having at least a predetermined energy density gradient on amorphous silicon. By employing visible laser light of lower absorption coefficient on amorphous silicon, this technique provides a limited temperature gradient in the film thickness direction while intentionally forming temperature gradient in the width direction to cause one dimensional lateral growth. This leads to a polycrystalline silicon film having a crystal array of a large grain size.
Patent Document 1: Japanese Patent Laying-Open No. 2000-286195